In situ combustion process



United States Patent 3,055,422 IN SITU COMBUSTION PROCESS Arthur R. Schleicher, Urbaua, Ill., and Joseph C. Trantham, Bartlesville, Okla, assignors to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Oct. 16, 1958, Ser. No. 767,507 9 Claims. (Cl. 166-11) This invention relates to an improved process for the recovery of hydrocarbons from a carbonaceous stratum by in situ combustion.

In situ combustion in the recovery of hydrocarbons from underground strata containing carbonaceous material is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to move thru the stratum by either inverse or direct air drive whereby the heat of combustion of a substantial proportion of the hydrocarbon in the stratum drives out and usually upgrades a substantial proportion of the unburned hydrocarbon material.

The ignition of carbonaceous material in a stratum around a borehole therein followed by injection of air thru the ignition borehole and recovery of product hydrocarbons and combustion gas thru another borehole in the stratum is a direct air drive process for effecting in situ combustion and recovery of hydrocarbons from the stratum. In this type of operation the stratum usually plugs in front of the combustion zone because a heavy viscous liquid bank of hydrocarbon collects in the stratum in advance of the combustion zone which prevents movement of air to the combustion process. To overcome this difliculty and to permit the continued progress of the combustion zone thru the stratum, inverse air injection has been resorted to. By this technique, a combustion zone is established around an ignition borehole by any suitable means and air is fed thru the stratum to the cornbustion zone from one or more surrounding boreholes.

In situ combustion techniques are being applied to tar sands, shale, Athabasca sand and other strata in virgin state, to lignite and other coal veins by fracturing, and to strata partially depleted by primary and even by secondary and tertiary recovery methods.

It is an object, of the present invention to provide an improved process for the recovery of hydrocarbons from a carbonaceous stratum by in situ combustion. Another object is to increase the rate of advance of the combustion zone thru the stratum in such a process. A further object is to increase the percent of recovery of hydrocarbons from the stratum in such a process. It is also an object of the invention to provide a process for restarting an inverse in situ combustion process when temporarily halted. Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

We have found that the rate of movement of the combustion zone or front thru a carbonaceous stratum can be materially advanced and that the percentage of recovery of the carbonaceous material as valuable product can be increased in an inverse in situ combustion process by injecting into the stratum, along with the injected air, a minor proportion of a gaseous fuel such as normally gaseous hydrocarbons and mixtures thereof. The proportions of fuel gas injected with the air is in the range of 0.25 to 7% by volume based on the total volume of air and fuel gas injected.

The preferred procedure is to inject a premix of fuel gas and combustion-supporting, O -containing gas, such as air or oxygen-enriched air, into the injection wells utilized in an inverse in situ combustion process. It is usually preferred to use a stoichiometric excess of 0 relative to the fuel gas. However, the fuel gas and air may be separately injected thru separate wells, such as in the practice of injecting the air from an outer ring of injection wells while injecting the fuel gas thru an inner ring of injection wells surrounding a central well. In this type of operation the mixing is not so satisfactory as in the process wherein a premix of the air and fuel is injected. It is also feasible to utilize the process of the invention in the recovery of hydrocarbons by in-line drive of the combustion front from a series of in-line produc tion wells flanked on each side by a generally parallel line of injection wells as is conventionally practiced in the art.

The benefits derived from the injection of fuel gas along with the oxygen supply to a combustion front in an in situ combustion process are peculiar to the inverse air injection technique because the injected fuel gas and oxygen supply passes thru cool stratum on the way to the combustion front and arrives there in cool condition. No burning is effected in the stratum before the gases arrive at the leading edge of the combustion front, whereas, in a direct injection process, the injected gases pass thru the hot burned out stratum back of the combustion front and, hence, the fuel gas, if injected in this type of process, is consumed before arriving at the leading edge of the combustion front and has the effect of heating up the burned out stratum and reducing the available freeoxygen for propagating the lead edge of the combustion front, both of which effects are detrimental to the recovery process.

To illustrate the benefits of the invention, a crushed tar sand was packed into a 2 inch diameter stainless steel tube. In one in situ combustion run, air alone was passed thru the packed tube to the opposite end where a combustion front had been established so as to move the combustion zone thru the tube countercurrently to the flow of air. Another run, utilizing the same type of tar sand packing, was under the same conditions except that the injected gas consisted of air containing two volume percent of propane. The data obtained in the runs are presented in the table below.

It can be seen from the test data that the propagation rate was increased 25% and the oil yield was increased 912% by the injection of propane with the combustion supporting gas (air). These benefits are advantageous from the viewpoint of economics by both increasing the total hydrocarbon recovery and by shortening operation time.

Another advantage in injecting fuel along with the air is the prevention of premature reversal of the direction of travel of the combustion zone, sometimes caused by lack of fuel in the zone just ahead of the leading edge of the combustion zone. The technique is also useful in restarting the inverse combustion process in case the injection of air must be temporarily stopped for any reason, such as failure of compressors, etc. In restarting without injection of fuel, the combustion tends to reverse its direction and move back thru the carbonized residue in the partially burned out stratum thru which the combustion front has just traveled. With injection of fuel along with the air, the movement of the combustion front inversely, i.e., countercurrently to the injected gas is reestablished Without difficulty, it being necessary only that a combustion supporting temperature exists in the stratum in the former combustion area when the gaseous mixture of fuel and air arrives.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

We claim:

1. A process for producing hydrocarbons from a carbonaceous stratum comprising igniting said stratum along an elongated vertical section thereof around a production well therein so as to establish a vertically elongated combustion zone around said well; passing a premix of fuel gas and o -containing gas in which said fuel gas is in the range of 0.25 to 7 volume percent of said premix from an injection Well in said stratum thru an unburned section thereof into said combustion zone so as to burn said fuel gas in said elongated combustion zone and move same thru said stratum away from said production well toward said injection well and continuing to inject said premix and burning said fuel gas in said zone to move same thru said stratum, thereby producing more hydrocarbons from said stratum at a faster rate than is effected Without said fuel gas in said premix; and recovering said hydrocarbons thru said production well.

2. The process of claim 1 wherein said fuel gas comprises naturally gaseous hydrocarbon.

3. The process of claim 2 wherein said fuel gas comprises propane.

4. The process of claim 2 wherein said fuel gas comprises natural gas.

5. A process for producing hydrocarbons from a carbonaceous stratum comprising igniting a vertically elongated section of said stratum around a production well therein so as to establish a combustion zone around said well; injecting a premix of o -containing gas and fuel gas thru injection wells spaced from said production well, said fuel gas being in a concentration in the range of 0.25 to 7 volume percent of said premix; passing said premix thru said stratum to said combustion zone so as to burn said fuel in the outermost edge thereof and advance same thru said stratum substantially to said injection wells, thereby producing more hydrocarbons from said stratum at a faster rate than is effected wtihout said fuel gas in the injected gas; and recovering said hydrocarbons from said production well.

6. The process of claim 5 wherein said injection wells comprise a ring of wells surrounding said production well.

7. The process of claim 5 wherein a combustion zone is established around each well in a line of production wells and said injection wells comprise a parallel line of wells on each side of said line of production wells.

8. The process of claim 7 wherein said fuel gas comprises normally gaseous hydrocarbon.

9. In a process for producing hydrocarbons from a permeable carbonaceous stratum comprising moving an inverse burning vertically elongated combustion zone thru said stratum from a production well toward an injection well therein by injection of O -containing, combustionsupporting gas thru said injection well; and recovering produced hydrocarbons thru said production well, where in combustion in said zone is temporarily terminated for lack of O and re-establishing the supply of O to said zone thru said injection well before the temperature thereof falls below combustion level revives combustion but reverses its direction; the improvement comprising: re-establishing said inverse burning combustion zone by feeding thereto thru said injection well a premix of said O -containing gas and fuel gas, the latter being in a concentration in the range of 0.25 to 7 volume percent of said premix, so as to re-establish an inverse burning combustion zone.

References Cited in the file of this patent UNITED STATES PATENTS 2,642,943 Smith June 23, 1953 2,793,696 Morse May 28, 1957 2,880,803 Parker Apr. 7, 1959 2,901,043 Campion et a1 Aug. 25, 1959 

9. IN A PROCESS FOR PRODUCING HYDROCARBONS FROM A PERMEABLE CARBONACEOUS STRATUM COMPRISING MOVING AN INVERSE BURNING VERTICALLY ELONGATEE COMBUSTION ZONE THRU AND STRATUM FROM A PRODUCTION WELL TOWARD AN INJECTION WELL THEREIN BY INJECTION OF O2-CONTAINING, COMBUSTION SUPPORTING GAS THRU SAID INJECTION WELL; AND RECOVERING PRODUCED HYDROCARBONS THRU SAID PRODUCTION WELL, WHEREIN COMBUSTION IN SAID ZONE IS TEMPORARILY TERMINATED FOR LACK OF O2 AND RE-ESTABLISHING THE SUPPLY OF O2 TO SAID ZONE THRU SAID INJECTION WELL BEFORE THE TEMPERATURE THEREOF FALLS BELOW COMBUSTION LEVEL REVIVES COMBUSTION BUT REVERSES ITS DIRECTION; THE IMPROVEMENT COMPRISING: RE-ESTABLISHING SAID INVERSE BURNING COMBUSTION ZONE BY FEEDING THERETO THRU SAID INJECTION WELL A PREMIX OF SAID O2-CONTAINING GAS AND FUEL GAS, THE LATTER BEING IN A CONCENTRATION IN THE RANGE OF 0.25 TO 7 VOLUME PERCENT OF SAID PREMIX, SO AS TO RE-ESTABLISH AN INVERSE BURNING COMBUSTION ZONE. 